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ORIGINALARTICLE
Historical patterns of oak populationexpansion in the Chautauqua Hills,KansasThomas R. Rogers and F. Leland Russell*
Department of Biological Sciences, Wichita
State University, Wichita, KS 67260, USA
*Correspondence: F. Leland Russell,
Department of Biological Sciences, Wichita
State University, Wichita, KS 67260, USA.
E-mail: [email protected]
ABSTRACT
Aim Rates of tree population expansion have increased in many North Ameri-
can landscapes that were mosaics of grasslands, savannas and woodlands his-
torically. Consequences of woodland expansion include reduced economic
return from grazing and changes in native biodiversity, but causes of woodland
expansion are poorly understood. We address historical timing of blackjack
oak (Quercus marilandica) and post oak (Quercus stellata) population expan-
sion in tree–grass ecosystems, roles of climate and land use in driving this
expansion, and future stability of these woodlands.
Location The Cross Timbers ecosystem in Kansas, USA.
Methods Using increment cores, we quantified blackjack oak and post oak
age structures in four woodlands on sites that were not wooded in the 1860s.
We compared timing of oak regeneration with climate fluctuations (using the
Palmer drought severity index) and land-use history. We quantified tree species
composition within 5-m radii of sampled oaks.
Results Recruitment of both oak species increased between 1925 and 1945.
Modal age classes recruited from 1935 to 1960. Recruitment was associated
with dry intervals at the two sites with north- or east-facing aspects. This asso-
ciation was driven by blackjack oak recruitment in dry intervals. Woodlands
on the sites with south- and west-facing aspects contained only oaks, whereas
those on the sites with north- and east-facing aspects contained saplings of
fire-intolerant, shade-tolerant tree species.
Main conclusions Our results contribute to growing evidence for woodland
expansion in the region during dry climate intervals. The association between
drought and recruitment was influenced by slope aspect and was more pro-
nounced in the less fire-tolerant oak species. Although woodland expansion
coincided with regional increases in fire frequency, drought and greater use of
prescribed burning are likely to have reduced fire intensities by reducing fuel
loads. These oak woodlands, which have developed during the 20th century,
appear to form stable communities on xeric slopes but to be undergoing suc-
cession towards a mesophytic tree community on mesic slopes.
Keywords
Age structure, Cross Timbers, fire ecology, Kansas, mesophication, Quercus
marilandica, Quercus stellata, tree recruitment, woodland dynamics, woody
plant encroachment.
INTRODUCTION
Landscapes that are mosaics of grasslands, savannas and
woodlands typically are dynamic, with the physiognomy of
individual patches changing on time-scales from decades to
centuries (Callaway & Davis, 1993; Moustakas et al., 2009).
Such shifts in physiognomy can be associated with substan-
tial changes in ecosystem functioning, such as changes in
ª 2014 John Wiley & Sons Ltd http://wileyonlinelibrary.com/journal/jbi 2105doi:10.1111/jbi.12360
Journal of Biogeography (J. Biogeogr.) (2014) 41, 2105–2114
hydrological and biogeochemical processes (Archer et al.,
2001; Huxman et al., 2005), and in ecosystem structure, such
as levels of native biodiversity (Ratajczak et al., 2012). While
transitions among all three physiognomies occur in land-
scape mosaics, rates of woody plant establishment in grass-
lands and savannas have increased substantially within the
last 50–300 years, with examples from all continents except
Antarctica (Archer et al., 1988). Globally, humans use tree–
grass ecosystems extensively for raising livestock, and there-
fore shifts from grassland or savanna to woodland are of
economic interest because of the reduction in forage (Bok-
dam et al., 2000; Dube & Pickup, 2001). Although causes of
woody plant expansion remain difficult to identify, contrib-
uting factors include changes in human land use (McPherson,
1997; Hessl & Graumlich, 2002; Abrams, 2003) and climate
changes (Archer et al., 2001) that alter tree–grass
interactions.
Fire and grazing influence strengths and outcomes of
tree–grass competitive interactions and, hence, can deter-
mine rates of woody plant encroachment. High-intensity,
frequent fires favour herbaceous vegetation and exclude
woody plants because most juvenile woody plants are sus-
ceptible to fire (Knapp et al., 2009). Rangeland management
may involve prescribed fire where fire suppression and habi-
tat fragmentation have eliminated wildfires, but managed
burns are often conducted when fuel loads are low, produc-
ing fires that are not sufficiently intense to suppress woody
plant recruitment (Knapp et al., 2009). Grazing can have
complex direct and indirect effects on woody plant–grass
interactions. Some of these effects may inhibit woody plant
establishment, such as the consumption of seedlings and
increased evaporation from the soil surface, and others may
promote woody plant establishment, such as reduced herba-
ceous competition and less fuel for fires (Riginos & Young,
2007; Cipriotti & Aguiar, 2012). Where cattle are the princi-
pal grazers, increased grazing is hypothesized to promote
woody plant establishment because cattle do little browsing
(Riginos & Young, 2007).
Climate fluctuations can play a critical role in woodland
expansion into grasslands and savannas. High precipitation
and low evapotranspiration can alleviate grass–woody plant
belowground competition and reduce fire frequencies and
intensities by keeping fuels moist. These mechanisms under-
lie a long-standing hypothesis that woody plant recruitment
will be greater during wet, cool climates (Archer et al., 1988;
McPherson & Wright, 1990). Recent results, however, suggest
an association between drought and woody plant recruitment
in pyrogenic grassland–savanna–woodland landscapes (Zie-
gler et al., 2008; Shuman et al., 2009). Low soil moisture
availability may reduce herbaceous biomass production,
reducing fire intensities and allowing woody plants to survive
fires (Ziegler et al., 2008). The ecological conditions under
which moisture and drought are each associated with woody
plant population expansion remain poorly understood.
In relatively mesic grassland–savanna–woodland land-
scapes, sustained changes in fire regimes may not only allow
woodland expansion, but may also promote woodland suc-
cession to forests dominated by fire-intolerant, shade-tolerant
tree species. Nowacki & Abrams (2008) refer to this shift
among alternate stable states in woodlands as ‘mesophica-
tion’. Throughout much of the eastern deciduous forest
region, oak forests are undergoing succession as fire-intolerant,
shade-tolerant tree species come to dominate the understo-
rey and densities of oak saplings and small adults are
reduced. Nowacki & Abrams (2008) hypothesized that oak
woodlands may be a stable community in eastern North
America only on xeric sites. Nevertheless, recent research in
the Oklahoma Cross Timbers region suggests replacement of
oaks by eastern red cedar (Juniperus virginiana) and calls
into question the stability of oak woodlands at the western,
xeric limit of eastern deciduous forest in North America
(DeSantis et al., 2011).
The Cross Timbers ecosystem in Texas, Oklahoma and the
Chautauqua Hills of south-eastern Kansas, USA, represents
the western and southern portion of the boundary between
eastern deciduous forest and prairie in North America. This
ecotone historically was a mosaic of grassland, savanna and
woodland (Dyksterhuis, 1949). Blackjack oak (Quercus mari-
landica Muenchh.) and post oak (Quercus stellata Wang.) are
the dominant tree species in the Cross Timbers. These spe-
cies belong to separate oak subgenera that differ in drought-
and fire-tolerance (Johnson et al., 2009). In the Chautauqua
Hills comparisons of modern plant communities with vegeta-
tion data from General Land Office (GLO) records from the
1860s suggest that tree cover has increased. The ecosystems
of the Chautauqua Hills have experienced profound land-use
changes and large climate fluctuations over the past
150 years that may have driven regional change in vegetation
physiognomy.
Here we quantify age structures of blackjack oak and post
oak populations to examine the historical timing and causes
of oak expansion in the Chautauqua Hills, Kansas, USA, and
to assess future successional change in these woodlands. We
address the following questions. (1) When did oak expansion
occur? (2) Do blackjack oak and post oak exhibit differential
timing of expansion? (3) Did oak expansion coincide with
climate fluctuations (drought versus high water availability)
or changes in land-use and fire regime? (4) Are these oak
woodlands stable or are they undergoing succession towards
a mesophytic tree community?
MATERIALS AND METHODS
Study species
Blackjack oak belongs to the subgenus Erythrobalanus (red
oak). Blackjack oaks are deciduous trees that grow 6–9 m
tall. Their range extends from New Jersey west to Iowa, east-
ern Kansas and central Texas and south to north-western
Florida (Harlow et al., 1996). In Oklahoma woodlands,
blackjack oaks of up to 120 years old occur (DeSantis et al.,
2011).
Journal of Biogeography 41, 2105–2114ª 2014 John Wiley & Sons Ltd
2106
T. R. Rogers and F. L. Russell
Post oak belongs to the subgenus Leucobalanus (white
oak). Post oaks are deciduous trees that reach 15–21 m in
height. This species occurs from southern New England west
to eastern Kansas and central Texas and south to central
Florida. Post oak is relatively drought-tolerant, but less so
than blackjack oak. In a literature review of factors affecting
oak regeneration in eastern North America, McQuilken
(1983) ranked blackjack oak as most drought-tolerant and
post oak as second-most drought-tolerant among seven
upland oak species in central Pennsylvania. Post oaks, as is
true for most species in the white oak subgenus, form tyloses
in fire-damaged vascular tissue, making them more fire-tolerant
than most species in the red oak subgenus (Johnson et al.,
2009). Post oak lifespans in the Cross Timbers can exceed
300 years (Therrell & Stahle, 1998).
Study sites
Criteria used to select study sites were (1) presence of both
blackjack oak and post oak, and (2) evidence of increased
tree cover in comparison with GLO records. Four sites were
selected: Cross Timbers State Park (37°440 N, 95°560 W;
hereafter ‘Cross Timbers SP’); Fall River State Lake
(37°390 N, 96°020 W; hereafter ‘Fall River’); Woodson State
Fishing Lake (37°470 N, 95°500 W; hereafter ‘Woodson’); and
Stotts’ Ranch (37°300 N, 96°010 W; hereafter ‘Stotts’). All
sites are on sandstone-derived soils. Fall River and Stotts are
in the westernmost counties in which blackjack oak and post
oak are recorded in Kansas (Great Plains Flora Association,
1977). Cross Timbers SP and Woodson are one county east
of these western range limits.
Cross Timbers SP and Woodson have been managed by
the Kansas Department of Parks and Wildlife since 1960 and
1937, respectively. Fall River has been managed by the Army
Corps of Engineers since 1949. At all three sites, prescribed
burning is conducted in a 3-year rotation with spot burning
to control invasion of woody plants and non-native species.
Stotts is privately owned. Between the 1930s and 1970s the
ranch was grazed heavily (Caleb Stotts, pers. comm.). Since
2000, stocking rates have been at or below recommended
National Resources Conservation Services levels. The current
prescribed burn strategy (2007–2011) involves burning
between two and four out of five years (Caleb Stotts, pers.
comm.), but historically burn strategies have varied. Guyette
et al. (2011) determined fire history for this site using tree
rings.
In GLO records, no timber is reported in the quarter sec-
tions where we sampled at Cross Timbers SP, Fall River and
Stotts. The vegetation mosaic at Woodson was more com-
plex. Surveyor notes for the sections that included our sam-
ple site at Woodson indicate that between 25% and 70% of
the lengths of the section boundaries were woodland, with
the remainder described as prairie. GLO surveys relevant to
Cross Timbers SP and Woodson were conducted in 1859
and surveys relevant to Fall River and Stotts were conducted
in 1867.
Field methods
Within each site, discrete woodland patches were defined by
natural or human-made boundaries (e.g. drainages, roads,
fences). Patch accessibility and size were considered in select-
ing one woodland patch to sample per site. At all four sites,
selected woodlands were on uplands that included first- and
second-order intermittent stream drainages, but not peren-
nial streams. Stands sampled at Cross Timbers SP and Fall
River occupied north- and east-facing aspects, while stands
at Stotts and Woodson occupied south- and west-facing
aspects. Using Google Earth, we determined the distance
between the northern- and southern-, and eastern- and wes-
tern-most points of each selected woodland. Maximum dis-
tances along north–south, east–west axes were used to
establish a coordinate system. We randomly selected a point
on this coordinate system to begin sampling trees. From this
point, we ran 100-m transects in the four cardinal directions.
The point-quarter method of sampling was used at 20-m
intervals along each of the four transects. At each 20-m
increment, the nearest blackjack oak and post oak tree was
sampled within each of four quadrants (80 trees species�1
site�1). If a transect exited the woodland before reaching
100 m, sampling on the next transect was extended to com-
pensate. Upon encountering a cluster of stems, possibly the
same genet, one stem was randomly selected for sampling.
Stems were considered to be the same genet if one stem
sprouted from the base of another, they were ≤ 30 cm apart,
or they were arranged in a circle and leaned away from each
other.
For each sampled tree, we measured its basal diameter, its
diameter at breast height (d.b.h.) and extracted an increment
core from the base of trees ≥ 4 cm d.b.h. If the increment
core collected from a tree did not pass through the centre of
the trunk then that tree was excluded from the age structure
and all analyses involving age. Therefore, actual sample sizes
were less than the 80 trees species�1 site�1 that our sampling
design should have provided. The minimum sample size
upon which age structures and analyses involving tree ages
were based was 41 trees, which occurred for blackjack oak at
Fall River and also at Cross Timbers SP.
To gain insight into spatial patterns of woody plant
expansion in uplands, we were interested in the relationship
between tree age and landscape features such as slope steep-
ness and topographic position. Therefore, for each tree sam-
pled we recorded slope position (ridge, mid-slope or
drainage) and measured slope steepness at the tree’s base
using a clinometer. To quantify tree species composition at
the sampling sites, the species and d.b.h. of all trees within
5 m of each sampled oak in the north-east quadrant at each
sampling point were recorded.
The samples that we collected to quantify age structures of
the two oak species included few trees that occurred in
drainages. Therefore, to evaluate the relationship between
topographic position and tree age, we augmented the sample
of trees in drainages at each site. We extended a pre-existing
Journal of Biogeography 41, 2105–2114ª 2014 John Wiley & Sons Ltd
2107
Oak population expansion in south-eastern Kansas
transect towards the nearest drainage where we randomly
determined which direction (upstream or downstream) and
which side of the drainage would be sampled. A 100-m tran-
sect was placed 10 m from the edge of the stream bed, and
at 20-m intervals along the transect sample trees were ran-
domly selected by drawing a piece of paper (labelled NE, SE,
NW or SW) corresponding to a quadrant in which the near-
est tree of each species would be cored. A total of five cores
from drainages were collected for each species at a site.
Across all four sites, 40 additional cores were sampled using
this method and were included in the landscape position
analysis.
Sampling occurred from June 2010 to July 2011. Incre-
ment cores were allowed to dry for ≥ 96 h, mounted, and
sanded with progressively finer (160–400 grit) paper. Ring
widths were measured using Coo Recorder (Cybis Elek-
tronic & Data AB, Saltsjobaden, Sweden) and were analysed
by visual cross-dating. Visual cross-dating of cores was con-
firmed using CDendro (Cybis Elektronic & Data AB) and
cofecha (Holmes, 1994). These programs are used to elimi-
nate age-related variation in radial growth rates, and to
detect missing or false rings.
Statistical analysis
We compared population age structures between the two
oak species at each site by using a Kolmogorov–Smirnov
two-sample test. To specifically compare mean age and mean
d.b.h. between the species at each site we used one-way
ANOVAs. Throughout our analyses, when multiple compari-
sons were made, as for separate Kolmogorov–Smirnov two-
sample tests and ANOVAs for each site, we used Bonferroni
adjustments to significance levels. Statistical analyses were
conducted using SAS Enterprise Guide 4.2.
To determine the historical landscape position of oaks, we
examined relationships between (1) tree age and slope steep-
ness, and (2) tree age and topographic position. We pooled
the oak species for these analyses. The relationship between
slope steepness and tree age was tested with an ANCOVA
using steepness as a covariate and site as a random effect.
For each site, we compared oak age among topographic posi-
tions (ridge, mid-slope and drainage) using ANOVA. Age
was square-root transformed for ANCOVAs and ANOVAs to
improve normality of the residuals and homoscedasticity.
To evaluate the relationship between water availability and
oak recruitment, we compared the proportion of trees at
each site that recruited in wet versus dry 7-year climate
intervals with the proportion of such intervals in the 1898–
2008 (the most recent year when a tree that we cored
recruited) climate record that were wet versus dry, as in Zie-
gler et al. (2008). We obtained Palmer drought severity index
(PDSI) values for Kansas climate region 9, which includes
our four study sites (NOAA, 2013). The year in which a tree
recruited was designated as part of a ‘wet’ climate interval if
the 7-year average PDSI, calculated using PDSI values for
3 years preceding the year of recruitment, the year of recruit-
ment and 3 years following the year of recruitment, was > 0.
If the 7-year average PDSI was < 0 then the year of recruit-
ment was designated as occurring in a ‘dry’ climate interval.
We used a 7-year running average of PDSI values in our
analyses because Ziegler et al. (2008) found PDSI 7-year run-
ning averages to predict timing of burr oak (Quercus macro-
carpa) regeneration at the eastern deciduous forest–prairie
ecotone in Minnesota, USA. First, for each site we used a
likelihood ratio test to determine whether blackjack oak and
post oak differed in the proportion of trees that recruited in
wet versus dry intervals. We then combined the species and
used likelihood ratio tests to compare the proportion of trees
that recruited in wet versus dry intervals with the proportion
of intervals that were wet versus dry for each site.
We used tree species composition within 5-m radii of oaks
that we sampled in the north-eastern quadrant at each sam-
pling point to quantify representation of mesophytic tree
species in the woodland. At sites where these 5-m radius cir-
cles included mesophytic species, we compared d.b.h. among
tree species using one-way ANOVA followed by Tukey tests.
RESULTS
Size structures
Diameter at breast height ranged from 3.2 to 52.3 cm for
both species across all four sites. The modal d.b.h. class for
both oak species was 15–20 cm at Cross Timbers SP, Fall
River and Stotts. At Woodson, the modal d.b.h. class was
10–15 cm for blackjack oak and 25–30 cm for post oak. The
oak species only differed in d.b.h. at Woodson, where post
oak was larger (F1,120 = 17.20; P < 0.001).
Age structures
Age distributions for the two oak species differed signifi-
cantly at Woodson (D = 0.498, P < 0.001) only. Mean age
of individuals sampled for the two species differed signifi-
cantly at two of the four sites, but the identity of the species
with the greater mean age differed among these two sites
(Table 1). Post oak was older at Woodson (F1,106 = 12.24;
P < 0.001) and blackjack oak was older at Stotts
(F1,107 = 6.47; P = 0.012). Mean age of post oak was margin-
ally significantly greater than blackjack oak at Cross Timbers
SP (F1,88 = 5.63; P = 0.020).
Post oak age structures were unimodal at Cross Timbers
SP, Fall River and Stotts (Fig. 1) with modal age classes of
65–70 years, 50–55 years and 25–30 years, respectively. The
post oak age structure at Woodson was bimodal with the
modal age class being 60–65 years and the second most fre-
quent age class being 145–150 years. Only the post oak pop-
ulation at Woodson included trees that were present before
GLO surveys. Four trees recruited from 1819 to 1850. Age
structures of blackjack oak were unimodal at all four sites
(Fig. 1), with modal age classes between 40 and 60 years at
all sites (Table 1).
Journal of Biogeography 41, 2105–2114ª 2014 John Wiley & Sons Ltd
2108
T. R. Rogers and F. L. Russell
Landscape position effects
There was a marginally significant, negative relationship
between tree age and slope steepness (F1,3 = 9.88, P = 0.052).
However, this relationship was driven by three trees (out of
437 trees in the analysis) that were young and occurred on
the steepest slopes. Mean tree age did not differ between
drainages, mid-slopes and ridges at any site (P > 0.130).
Association with climate fluctuations
At Cross Timbers SP, blackjack oak recruitment was signifi-
cantly more associated with dry 7-year climate intervals than
was post oak recruitment [2lnL (likelihood ratio test statis-
tic) = 6.796, P = 0.009; Fig. 2]. The proportions of blackjack
oak and post oak recruiting during dry intervals at Cross
Timbers SP was 0.66 (SE 0.074) and 0.46 (SE 0.066), respec-
tively. Higher proportions of blackjack oaks than of post
oaks recruited during dry intervals at Fall River [blackjack:
0.61 (SE 0.076); post: 0.52 (SE 0.062)], Stotts [blackjack: 0.49
Table 1 Summary of descriptive statistics for blackjack oak
(Quercus marilandica) and post oak (Q. stellata) age structuresat the four study sites in the Chautauqua Hills, Kansas. At each
site, trees were sampled using the point-quarter method alongfour 100-m transects. The minimum diameter at breast height
(d.b.h.) for sampling was 4 cm.
Site Species
Age
range
(years)
Mean
(� SE)
age (years)
Modal
age class
(years)
% of
trees in
modal
age class
Cross
Timbers
Blackjack oak 17–86 57.0 (� 2.2) 50–55 26.8
Post oak 26–138 66.7 (� 2.9) 65–70 25
Fall River Blackjack oak 11–115 55.6 (� 2.9) 55–60 19
Post oak 5–148 57.9 (� 3.2) 50–55 17.6
Stotts Blackjack oak 11–83 41.1 (� 2.3) 40–45 15.7
Post oak 4–68 33.7 (� 1.6) 25–30 21.4
Woodson Blackjack oak 4–112 41.4 (� 3.2) 45–50 24
Post oak 6–193 80.5 (� 6.7) 60–65 13.6
Figure 1 Post oak (Quercus stellata) andblackjack oak (Q. marilandica) age
structures at the four Cross Timberswoodlands, Kansas. Five-year age categories
are used. At each site, trees were sampledusing the point-quarter method along four
100-m transects. The minimum diameter atbreast height (d.b.h.) for sampling was
4 cm.
Journal of Biogeography 41, 2105–2114ª 2014 John Wiley & Sons Ltd
2109
Oak population expansion in south-eastern Kansas
(SE 0.070); post: 0.40 (SE 0.058)] and Woodson [blackjack:
0.57 (SE 0.071); post: 0.50 (SE 0.077)], but these differences
in recruitment between the oak species were not statistically
significant.
After pooling the two species, oak recruitment was signifi-
cantly associated with dry intervals at Fall River (2lnL = 7.144,
P = 0.007; Fig. 2). The proportion of trees of both oak species
recruiting during dry climate intervals at Fall River was 0.56
(SE 0.048) and the frequency of years that were in the middle
of dry 7-year intervals in the 111-year regional PDSI record
was 0.42 (SE 0.047). After Bonferroni correction for separate
tests at the different sites, there was a marginally significant
association of oak recruitment with dry intervals at Cross Tim-
bers SP (2lnL = 5.096, P = 0.020; Fig. 2) where the proportion
of oak trees recruiting during dry intervals was 0.54 (SE 0.05).
This marginally significant association was driven by a very
strong association of blackjack oak recruitment and drought
(2lnL = 8.880, P = 0.003), whereas post oak recruitment was
associated with neither dry nor wet conditions (P = 0.661). At
Stotts and Woodson, oak recruitment was not associated with
dry or wet intervals. The proportion of oak trees recruiting
during dry intervals was 0.44 (SE 0.044) and 0.54 (SE 0.052) at
Stotts and Woodson, respectively.
Tree species composition
The two woodlands on mesic slopes, Fall River and Cross Tim-
bers SP, had greater tree species richness than the woodlands
on xeric slopes, Woodson and Stotts (Table 2). Only oaks were
encountered at Woodson and Stotts. Stem density of post oak
Figure 2 Climate fluctuation andrecruitment of blackjack oak (Q.
marilandica) and post oak (Q. stellata) atCross Timbers State Park and Fall River,
Kansas. The top panel presents 7-year(3 years before and after the year of tree
recruitment) running averages for Palmerdrought severity index values. At each site,
trees were sampled using the point-quartermethod along four 100-m transects. The
minimum diameter at breast height (d.b.h.)for sampling was 4 cm.
Journal of Biogeography 41, 2105–2114ª 2014 John Wiley & Sons Ltd
2110
T. R. Rogers and F. L. Russell
was greater than all other tree species at every site except for
Stotts, where blackjack oak had the greatest density. At Cross
Timbers SP black cherry (Prunus serotina), eastern red cedar
and eastern redbud (Cercis canadensis) had higher stem densi-
ties than blackjack oak. At Fall River, black cherry, eastern red
cedar and eastern redbud were present, but at lower stem den-
sities than post oak and blackjack oak. At Cross Timbers SP,
mean d.b.h. values for post oak and blackjack oak were signifi-
cantly greater than the mean d.b.h. of any other tree species
(P < 0.001) except American elm (Ulmus americana). At Fall
River, mean d.b.h. values for post oak and blackjack oak were
significantly greater than for any other species (P < 0.001).
DISCUSSION
Temporal patterns of oak regeneration
Our results demonstrate prolific oak recruitment in the Chau-
tauqua Hills in the early- to mid-20th century. We suggest that
this episode of oak recruitment at our study sites represents
initial expansion of woodlands into patches of the landscape
mosaic that were savanna or grassland. It seems unlikely that
the recruitment wave that occurred in the early to mid-20th
century was stimulated by harvesting woodlands that grew
after GLO surveys in the 1860s. In agricultural schedules from
the 1925 Kansas state census no respondents reported income
in the previous year from marketing wood in the townships
that include Stotts (Painterhood township) and Woodson
(Belmont township). Only 1 of 138 respondents and 2 of 143
respondents marketed wood in the townships that include Fall
River (Salt Springs township) and Cross Timbers SP (Toronto
township), respectively. For these four townships, Kansas cen-
sus agricultural schedules show peaks in the proportion of
respondents that marketed wood in 1885 (6.9%, Painterhood),
1895 and 1905 (8.8%, Salt Springs), 1905 (8.8%, Toronto) and
1915 (15.4%, Belmont). Harvesting in the late 19th and early
20th centuries probably did not involve woodlands that
expanded into grasslands and savannas in the interval since
GLO surveys were conducted because such trees would have
been young and presumably small. Young woodlands at our
study sites could have been cut without marketing the wood,
but trends in marketing wood probably reflect general land use
practices with regard to woodlands.
In the oak woodlands at our study sites, age distributions
of blackjack oak populations and post oak populations were
generally similar, but subtle differences in age structures
appear to correlate with differences between the species’ phy-
siologies and life histories. Species in the red (Erythrobal-
anus) and white (Leucobalanus) oak subgenera differ in
drought tolerance, fire tolerance and lifespan (Johnson et al.,
2009). For blackjack oak and post oak, mean age of trees
differed at two sites, and marginally so at a third. However,
because the identity of the species with the greater mean age
differs between the sites, differences in mean age are unlikely
to reflect a successional sequence among the oak species, but
instead reflect the longer lifespan of post oak. Only at the
site with the youngest oak populations (Stotts) did blackjack
oak have a greater mean age than post oak, whereas at the
sites with the oldest oak populations (Cross Timbers SP and
Woodson) mean age of post oak was greater. We suggest
that populations of either species can predominate in the
early stages of woodland expansion or their spread can occur
Table 2 Description of tree community composition at the four study sites in the Chautauqua Hills, Kansas. Species composition data
were collected within circles of 5-m radii around oaks sampled for age structures and do not include the oak at the centre of the circle.All stems taller than breast height were recorded. ‘Relative stand proportions’ refers to the proportion of the total density of trees
contributed by each species. If no standard error is shown after mean diameter at breast height (d.b.h.) then only one individual of thespecies was sampled.
Site
Species
richness Species composition
Mean (� SE) d.b.h.
(cm)
Tree density
(stems/ha)
Relative stand
proportion
Cross Timbers 6 Post oak (Quercus stellata) 14.11 � 0.66 133.96 0.33
Blackjack oak (Q. marilandica) 15.89 � 2.21 28.72 0.07
Eastern redubud (Cercis canadensis) 3.52 � 0.45 36.36 0.09
Eastern red cedar (Juniperus virginiana) 4.43 � 0.66 70.8 0.17
Black cherry (Prunus serotina) 3.86 � 0.64 120.56 0.30
American elm (Ulmus americana) 11.13 � 1.65 9.56 0.02
Fall River 7 Post oak 17.27 � 0.82 280.84 0.70
Blackjack oak 23.13 � 2.44 51.08 0.13
Eastern redbud 4.97 � 1.53 29.80 0.07
Sugarberry (Celtis laevigata) 3.50 4.24 0.01
Eastern red cedar 3.55 � 0.38 34.04 0.09
Osage orange (Maclura pomifera) 4.46 4.24 0.01
Black cherry 8.03 � 2.05 21.28 0.05
Stotts 2 Post oak 18.04 � 0.97 197.48 0.49
Blackjack oak 20.28 � 0.94 202.52 0.51
Woodson 3 Post oak 16.58 � 1.17 262.08 0.66
Blackjack oak 10.67 � 0.94 101.16 0.25
Red oak (Quercus rubra) 19.51 � 2.84 36.80 0.09
Journal of Biogeography 41, 2105–2114ª 2014 John Wiley & Sons Ltd
2111
Oak population expansion in south-eastern Kansas
simultaneously, but as woodlands age blackjack oaks die and
older age classes become dominated by post oak.
Our results also provide evidence of differences, perhaps
subtle, in regeneration niche between these two dominant
oak species of Cross Timbers woodlands. At Cross Timbers
SP blackjack oak recruitment was significantly more associ-
ated with dry climate intervals than was post oak recruit-
ment. Further, although differences at no other site reached
statistical significance, at all sites the proportion of blackjack
oak that recruited during dry intervals was greater than for
post oak. This difference in regeneration timing is consistent
with greater drought tolerance in blackjack oak and, poten-
tially, with less fire tolerance. Ziegler et al. (2008) and Shu-
man et al. (2009) hypothesized that woody plant population
expansion would be greatest during dry periods because
sparse herbaceous biomass would support less-intense fires.
Species in the red oak subgenus, in general, are less fire-tol-
erant than species in the white oak subgenus. Ultimately, dif-
ferences between the species in the degree of association
between dry conditions and recruitment may have had subtle
effects on age structures because between 1910 and 1960 dry
and wet periods of 10–15 years duration alternated regularly
in south-eastern Kansas (NOAA, 2013).
Correlates and possible causes of oak population
expansion
The relative synchrony of increased recruitment across our
study sites indicates that oak population expansion was initi-
ated by regional processes. We suggest that oak expansion in
the Chautauqua Hills is best explained by the combination
of drought, changes in natural fire regimes in the late 19th
century, and land management changes during the economic
and environmental crises of the 1930s.
Climate fluctuations are thought to be important in shifting
patch physiognomy in grassland–savanna–woodland landscape
mosaics (Archer et al., 1988; Ziegler et al., 2008). Although
woody plant encroachment into grasslands and savannas has
been hypothesized to coincide with moist climate in tree–grass
ecosystems, Ziegler et al. (2008) found that drought promoted
woodland expansion at the forest–prairie ecotone in Minne-
sota, USA. Our results also suggest an association between
drought and oak recruitment at our sites with mesic slopes,
Cross Timbers SP and Fall River. On mesic sites, we would
expect herbaceous biomass production to be greater than on
xeric sites and, hence, drought might be necessary to reduce
standing crop herbaceous biomass and fire intensity below a
threshold at which juvenile oaks can survive fire.
DeSantis et al. (2011) hypothesized that, historically,
drought allowed regeneration in oak woodlands of the Cross
Timbers and further west in Oklahoma. In existing woodlands,
severe droughts may kill adult oaks, increasing light penetra-
tion. Once drought abates, renewed moisture and high light
conditions in the understorey may provide an opportunity for
oak recruitment. Because GLO surveys for Cross Timbers SP
and Fall River do not indicate woodlands in the quarter sec-
tions that include our sites, we think that the mechanism pro-
posed by Ziegler et al. (2008) is more relevant for our sites.
Our results complement those of DeSantis et al. (2011) by
addressing the role of drought in oak regeneration, not in
existing woodlands, but in components of the historical land-
scape mosaic that were grassland or savanna.
Reduced herbaceous fuel loads associated with dry condi-
tions in the early to mid-20th century would probably have
reinforced reductions in fire intensities as prescribed fire was
increasingly used in land management. In contrast to less fre-
quent fires throughout much of eastern North America dur-
ing the 20th century (Nowacki & Abrams, 2008), fire
histories for Cross Timbers sites (Stambaugh et al., 2009;
DeSantis et al., 2010; Allen & Palmer, 2011; Guyette et al.,
2011) indicate that fires were more frequent in the 20th cen-
tury than in recent preceding centuries. Further, Guyette
et al. (2011) found that fire scars at Stotts occurred at more
frequent intervals than at any comparable site that they sam-
pled in the Great Plains. However, fires were probably less
intense in the Cross Timbers during the 20th century than
preceding centuries as a result of changes in the season of
fire (Allen & Palmer, 2011). Knapp et al. (2009) suggest that
before Euro-American settlement of the Great Plains, fires
occurred during both the growing and dormant seasons.
During the 20th century and today, prescribed burns are typ-
ically conducted in late winter or early spring in the Chau-
tauqua Hills (Guyette et al., 2011). Annual early spring fires
did not kill established shrubs, but instead allowed their slow
expansion in cover (Heisler et al., 2003) at Konza Prairie
Long Term Ecological Research site in the Kansas Flint Hills.
Further, Heisler et al. (2003) found that early spring burning
at a 4-year interval permitted extensive establishment of new
shrubs from seed. We suggest that less intense, even if more
frequent, fires created conditions under which oak popula-
tions could expand at our study sites.
In concert with reduced fire intensities as a result of
drought and changes in fire season, large changes in land use
in the 1930s may have allowed oak population expansion.
Economic depression in the United States in the 1930s
forced many Kansas residents to sell land or forfeit land to
banks, leaving land unmanaged for prolonged periods
(Hornbeck, 2012). Further, positive attitudes towards trees as
a means to limit erosion during the 1930s ‘Dust Bowl’ may
have accelerated woody plant establishment. For example,
the Prairie States Forestry Project (1935–42) planted > 217
million trees and more than 28,000 km of shelterbelt
throughout the Great Plains (Droze, 1977). In this context,
oaks may have been allowed to expand to create additional
erosion control and protection for pastures.
‘Mesophication’ of Chautauqua Hills oak woodlands
Oak recruitment began to decline at our study sites between
1960 and 1990 and low recruitment rates persist. Canopy
closure and low light availability probably contributed to
recent, limited recruitment. In xeric oak woodlands of wes-
Journal of Biogeography 41, 2105–2114ª 2014 John Wiley & Sons Ltd
2112
T. R. Rogers and F. L. Russell
tern North America, light limitation often results from self-
shading by adult oaks (Mensing, 1992). Oak recruitment
could resume after overstorey oaks die if juvenile oaks are in
the understorey. However, in eastern North America, light
limitation and failure of oak recruitment are associated with
the formation of dense understories of fire-intolerant, shade-
tolerant tree species that out-compete juvenile oaks for light
(Nowacki & Abrams, 2008). Nowacki & Abrams (2008)
hypothesized that oak woodlands may be a stable community
only on xeric sites in eastern North America. At the prairie–
deciduous forest ecotone in the Kansas Flint Hills, however,
even on steep, dry sites, declines in oak recruitment coin-
cided with increased recruitment of mesophytic tree species
(Abrams, 1986). Further, xeric oak woodlands in Oklahoma
do not appear to be stable as understories are dominated by
eastern red cedar, suggesting that when canopy oaks die they
will be replaced by eastern red cedars (DeSantis et al., 2011).
Our results suggest the hypothesis that slope aspect may
strongly influence the stability of oak woodlands in the
Chautauqua Hills. At our two sites with mesic, north- or
east-facing slopes, Cross Timbers SP and Fall River, meso-
phytic species, such as black cherry and fire-intolerant east-
ern red cedar, are well represented in the understorey.
Increases in mesophytic species in these woodlands are likely
to have occurred within the past 50 years. Hale (1955)
reported black cherry, hackberry (Celtis occidentalis) and
black walnut (Juglans nigra) only at very low densities in six
Chautauqua Hills oak woodlands that he sampled in 1954.
Oaks constituted > 97.5% of stems in all woodlands that
Hale (1955) sampled, whereas we found that 17% and 60%
of stems sampled were not oaks at Fall River and Cross Tim-
bers SP, respectively. Recent establishment of mesophytic tree
species at our study sites with mesic slopes is also indicated
by the much smaller diameters of these trees in comparison
with the oaks. By contrast, at our south- and west-facing
sites we encountered only oaks, suggesting greater stability at
these sites, consistent with Nowacki & Abrams’ (2008)
hypothesis. A note of caution, however, is required in assert-
ing the stability of oak woodlands on xeric slopes in the
Chautauqua Hills because fire is still used in management at
all of our study sites. These prescribed burns, however, only
burn into the edges of woodlands. For ecosystem restoration,
understanding where and when oak woodlands are stable is
important because savannas and open woodlands can be re-
established through thinning if oaks persist, but restoration
would be more challenging if a shift to a mesophytic alter-
nate stable state has occurred and oaks are lost entirely.
Landscape mosaics of grassland, savanna and woodland
are highly dynamic inherently. Understanding causes of shifts
in patch physiognomy within these mosaics and how human
land use influences such shifts has both economic and con-
servation implications. Our results support a growing body
of evidence for the counterintuitive conclusion that dry con-
ditions, even where tree species occur at their xeric range
limit, may allow woodland expansion. Further, our results
suggest the influence of aspect and identity of the woody
plant species in modifying effects of dry conditions on
woody plant population expansion. At this xeric range limit
for eastern oak species, aspect also appears to be important
in determining the stability of the oak woodlands that form.
Such results may enhance opportunities for spatially and
temporally targeted strategies to manage woody plant expan-
sion for economic and conservation goals.
ACKNOWLEDGEMENTS
We thank Randall Rogers for his consistent help with field-
work. We thank James Beck, Karen Brown, Greg Houseman,
Mary Liz Jameson, Oliver Keller, Maria Martino, Rob
McMinn, David Wickell and two anonymous referees for
insightful comments on early drafts of the manuscript. The
Stotts family, the Kansas Department of Parks, Wildlife and
Tourism, and the Army Corps of Engineers were of invaluable
assistance by providing access to study sites. We thank the
Kansas Historical Society for access to Kansas state censuses.
We are grateful to the Kansas Academy of Science Student
Research Grant Program and the High Plains Regional Climate
Center for funding.
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BIOSKETCHES
Thomas R. Rogers is interested in forest population and
community ecology. Recent research has focused on oak
population dynamics in woodlands and savannas, relation-
ships between landscape topography and tree species distri-
butions, and woody plant encroachment.
F. Leland Russell’s research focuses upon plant population
ecology and plant–animal interactions. Recent topics addressed
by Russell and colleagues include herbivore effects on plant pop-
ulation growth rates, spatial and temporal variation in herbivore
effects on host plants, tree population dynamics in woodlands
and savannas, and understorey restoration in oak savannas.
Editor: Miles Silman
Journal of Biogeography 41, 2105–2114ª 2014 John Wiley & Sons Ltd
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T. R. Rogers and F. L. Russell